The ideal neuroimaging technique would provide exquisite structural detail and also provide functional information, with high spatial and temporal resolution. Optical coherence tomography (OCT) is an optical imaging technique in which light from a low coherent source illuminates tissue and reflectivity of internal microstructures at different depths is measured by an interferometer. OCT is capable of micrometer-spatial and millisecond-temporal resolutions, without the use of exogenous contrast agents (hence label-free). The objective in this application is to develop and validate OCT for mammalian brain functional imaging; correlate OCT images with cellular electrophysiology assessed by multielectrode array (MEA); and provide proof-of- principle for OCT-based detection of neural activity. Two specific aims will be pursued: (1) Validate detection of multi-unit activity (MUA) by optical coherence tomography (OCT) in hippocampal slices. Our previous data demonstrate that OCT can detect synchronous cellular firing associated with both generalized and focal seizure activity. However, the sensitivity of OCT to physiological events such as multiunit activity (MUA) has not yet been determined. In this Aim, MUA induced by 4-aminopyridine (4-AP) and high K+ will be correlated to OCT. (2) Validate detection of local stimulation-induced synaptic activation by OCT. To allow more precise control of local stimulation site and intensity, in this Aim we will use local stimulation of a defined synaptic pathway in the hippocampal slice combined with OCT-based detection. Stimulation of the Schaffer collateral pathway from CA3 to CA1 will be performed by (1) electrical stimulation and (2) optogenetic stimulation to trigger MUA in CA1 that will be then detected by MEA and correlated to the changes in the optical signal by OCT. Our approach is innovative in adapting OCT for brain functional imaging. The proposed research is significant because it will lead to the validation of OCT as a neuroimaging tool for research in neuroscience.